Scrap-aluminum recycling starts with the melting of the scrap aluminum in open-hearth reverberatory furnaces. In order to decrease the interfacial tension of the dross/aluminum and allow the aluminum to coalesce for easier separation as well as to minimize the oxidation of the molten aluminum and to capture the aluminum oxide that forms during the melting process, a salt mixture called flux, is spread on top of the bed. Flux typically consists of about 62% by weight sodium chloride, 32% by weight potassium chloride and 6% by weight other materials, including cryolite (Na.sub.3 AlF.sub.6). This layer also serves to trap impurities such as magnesium chloride, which also reports to the flux layer. After the melting process is completed, the top layer of dark-colored material, called dross, is removed for further processing to recover some of its aluminum content. The black dross typically consists of aluminum metal (10-20% by weight), salt-flux mixture (40-55% by weight), and aluminum oxide (20-50% by weight). One of two methods is used to process the black dross for the recovery of aluminum: (1) reprocessing of the black dross in a rotary furnace (along with low grade scrap) or (2) physically separating the aluminum from the black dross using hammer mills. When the aluminum content of the dross is such that it is not economical to recover, the dross is disposed of in landfills. Rotary furnace processing requires the addition of salt fluxes in even larger quantities than that required for reverberatory furnace processing.
Residue generated by the rotary furnaces is referred to as salt cake, and it contains about 3-5% by weight aluminum, 15-30% by weight aluminum oxide, 30-45% by weight sodium chloride, 20-30% by weight potassium chloride and minor amounts of the chlorides of impurities such as magnesium chloride. The amount of black-dross and salt-cake residues generated from U.S. secondary aluminum-smelting plants in 1988 was approximately 229.times.10.sup.3 tons. The quantities of scrap and salt cake that are being processed are increasing. Because the salts are readily leachable, the disposal of this material in landfills is becoming more costly and less acceptable. Therefore, alternatives to disposal in landfills are in demand.